Heretofore, as immunological assays, enzyme immunoassays (EIA), immunoturbidimetric assays, and the like have been used. Among them, in recent years, immunoturbidimetric assays using insoluble carrier particles have been used widely in laboratory tests and the like because they are applicable to autoanalyzers.
The immunoturbidimetric assays usually use insoluble carrier particles carrying antigens or antibodies, which are measurement objects. When samples contain measurement objects, the measurement objects are bound to the antigens or antibodies carried by the insoluble carrier particles, and the insoluble carrier particles thereby are aggregated. Therefore, measurement objects can be detected by measuring the level of the aggregation.
In laboratory tests, disorders are diagnosed by measuring, for example, concentrations of antigens or antibodies contained in biological samples as diagnostic indices by the immunoturbidimetric assay. Since concentrations of antigens or antibodies contained in biological samples differ greatly depending on the presence or absence of disorders, for example, the inspection methods are required to be measurable over a wider range concentration.
Immunoturbidimetric assays using insoluble carrier particles can, from their measurement principles, widen the measurement concentration ranges of measurement objects by raising the upper limit of the amount of antibody or antigen that is reactable. To achieve this, insoluble carrier particles are required to have larger amount of antibody or antigen. However, with respect to insoluble carrier particles used in immunoturbidimetric assays, usually, when the insoluble carrier particles carry antibodies or antigens at functional groups on the surfaces of the particles, the electric charges of the surfaces thereof are decreased. This causes problems in which aggregation reactions of particles themselves and aggregation reactions (nonspecific aggregation) by substances other than antibodies or antigens occur easily, dispersibility is decreased, and measurement sensitivity is decreased.
Hence, with respect to the immunoturbidimetric assays, with the aim of suppressing nonspecific aggregation, a method of using proteins non-specific to antigens (Patent document 1), a method of using surfactants (Patent document 2), and a method of using salts (Patent document 3) have been developed. However, in the method of using proteins non-specific to antigens, when the amount of antibody or antigen to be sensitized with insoluble carrier particles is increased, the efficiency of suppressing nonspecific aggregation is decreased. Further, in the method of using surfactants, there is the possibility of losing reactivity because antibodies or antigens physically adsorbed are removed or denatured. Moreover, in the method of using salts, there is the possibility of decreasing reactivity and sensitivity of antibodies or antigens. According to these methods, the quality of insoluble carrier particles tends to vary depending on the type, lot, and the like of antibodies or antigens to be carried.    Patent document 1: JP 2004-117068A    Patent document 2: JP 11(1999)-258239A    Patent document 3: JP 2004-117022A